Slippery Slopes

In informal logic classes, instructors often teach students to avoid the "slippery slope fallacy". A "slippery slope" is an argument that a certain situation will lead to an even worse situation, and the first situation ought to therefore been avoided. Unfortunately, the use of slippery slopes does not necessarily entail a fallacy, and the fact that it is taught as such leads people to dismiss arguments that ought not to be dismissed. There are several uses of slippery slope arguments, and in this essay, I intend to explain the different uses of slippery slope arguments and I will argue that there is no special "slippery slope fallacy".

1) Reductio Slippery Slopes. The first use of slippery slope arguments is not a fallacy, but in fact a use of reductio ad absurdum argumentation. Reductios rely on the logical principle that anything that entails a falsehood must itself be false. For instance, when "if p, then q" is true and "q" is false, "p" must also be false. So, if all mammals are animals and my daffodil is not an animal, my daffodil must also not be a mammal.

In ethics, the reductio is used to show that someone's moral principles are inconsistent. If a set of moral principles leads to a conclusion that the person who holds those moral principles would not accept, he or she has inconsistent moral principles. An example of this is in recent gay marriage debates. If someone argues that gays should be able to marry since any two consenting, commited adults should be able to marry, this principle would also justify marriage of brothers and sisters. Since most people would not accept this conclusion, there must be some flaw in the original principles and they must either be changed or supplemented. Note that this argument does not imply that gay marriage would in fact lead to incestuous marriage, but it should in principle lead to incestuous marriage. No claim about what will happen in society is being made. The argument is being used to show inconsistency and is not being used fallaciously.

2) Empirical Slippery Slopes. Empirical slippery slopes are claims about facts, not claims about principles. In this case, one argues that a given situation will lead to a much worse situation in the future. For instance, one might argue that voluntary euthanasia among the elderly will lead to a devaluing of the lives of the elderly and will therefore lead to involuntary euthanasia among the elderly. Voluntary euthanasia among the eldery does not in principle require involuntary euthanasia; one can supplement the arguments that the lives of seriously ill people are not worth living with a principle respecting autonomy and avoiding paternalism.

Instead, an argument such as this must be demonstrated in the same way any empirical premise must be demonstrated, through evidence. In this case, the best approach would be to examine other countries that have allowed voluntary euthanasia and to see whether or not those countries have lapsed into involuntary euthanasia. If they have, the argument is a sound, empirical argument. If not, then the argument is unsound. If there are no countries that have done so or there is an insufficient time lapse, then one may use judgement about human nature while understanding the limitations of this sort of evidence. The argument becomes fallacious when an empirical slippery slope with insufficient evidence is asserted. However, this is not a special "slippery slope fallacy", but is just bad induction.

3) Identitative Slippery Slopes. These are not truly slippery slope arguments, but I have seen them referred to as such so it is worth mentioning. These are arguments that a current situation is just as bad as another situation that one's interlocutor sees as bad. It is not truly a slippery slope argument because the argument is not that something will lead to something worse, but that something the interlocutor already sees as worse is already happening. For instance, this kind of debate is raised relative to prenatal genetic testing for the purpose of selective abortion and eugenics. Most people accept that eugenics is bad, as a result of eugenics projects by Nazis and other groups. The argument is that prenatal genetic testing just is eugenics, practised not by selective breeding or sterilization, but through selective abortion using testing. This is not a fallacy, either. It attempts to show the interlocutor that, based on his or her own premises, a current situation is just as bad as another situation he or she believes to be worse. Like reductio slippery slopes, it is a demonstration of inconsistency in the interlocutor's own principles, which must be either modified or supplemented.

In conclusion, there is no special "slippery slope" fallacy. Rather, most slippery slope arguments are attempts to demonstrate inconsistencies in moral principles or make empirical claims about the future state of a society that allows certain things to occur. By teaching slippery slope arguments as fallacies, informal logic teachers are giving status to a facile defense of practices that may be either inconsistent or dangerous.